Large vehicles such as semi-trailer trucks and tractors often include or require a steerable suspension system. A steerable suspension system often improves the maneuverability of the vehicle by reducing its turning radius and providing a mechanism for the vehicle to maneuver around tight corners, for example by making small adjustments in the alignment of the vehicle wheels. In many instances, the steerable suspension system includes one or more auxiliary axles to enhance greater control over vehicle movement.
The term “steerable” suggests an ability to establish and maintain a desired caster angle of an axle, relative for example to an axis substantially perpendicular to the contact or driving surface of the vehicle. In particular, it is desirable to set the caster angle of an axle such that the wheels attached to the axle will not laterally deflect under self-energizing friction forces. Suspension systems known in the art provide a mechanism for establishing and maintaining a “positive” caster angle when the vehicle moves forward. Positive caster is a setting where the steering axis is inclined rearward at the top, as viewed from the side. Positive caster makes possible the self-centering force that tends to return the wheel to the direction the vehicle is traveling. Typically, a positive caster angle is on the order of five degrees.
An interesting problem arises, however, when a vehicle travels in a reverse direction to back up. Many prior art systems simply “lock” the caster angle/suspension system in the forward, or positive caster, position, thereby making it impossible for the wheels to move laterally as the vehicle travels in reverse. While effective at preventing lateral movement of the wheels, this approach reduces the overall maneuverability of the vehicle and eliminates advantages recognized with a steerable suspension system.
Other systems known in the art provide a mechanism for changing or reversing the caster angle by as much as five to ten degrees, thereby establishing a “negative” caster angle. By reversing the caster angle, the wheels accommodate the backward movement of the vehicle without lateral movement. Known systems for establishing both a positive and a negative caster angle are typically very complex, for example including multiple pivot arms and linkages coupled through a plurality of pivot points. These systems often require a plurality of air chambers and air bellows, and combinations thereof.
Structurally, the suspension systems known in the art require air chambers, air bellows or other actuation mechanisms that are positioned opposite one another. As can be appreciated, opposing components of the size and complexity of an air chamber or air bellows occupy a relatively large physical space in the suspension system. If the size of an air chamber or bellows is minimized for packaging purposes, often the mechanical force provided to rotate and maintain the caster angle is inadequate.